In a conventionally used semiconductor laser module, a laser diode (semiconductor laser) chip is optically coupled with an optical fiber. In this case, the optical axis of the optical fiber is aligned with that of the semiconductor laser and then the optical fiber is fixed onto a base using solder, adhesive, or the like.
FIG. 10 is a schematic view showing a conventional semiconductor laser module 100. The semiconductor laser module 100 has a semiconductor laser base 105 and a fiber-fixing base 109 that are arranged on a base 103 in alignment. A semiconductor laser 107 is fixed onto the semiconductor laser base 105. Also, an optical fiber 113 is fixed to the fiber-fixing base 109 using adhesive 111 or the like. In this state, the optical fiber 113 and the semiconductor laser 107 are optically coupled. Hereinafter, a fixing structure for an optical fiber and a fiber-fixing base will be called as a fixing structure for an optical fiber.
FIG. 11 (a) and FIG. 11 (b) show a fixing structure 110 for an optical fiber in the semiconductor laser module 100 wherein FIG. 11 (a) is a side view and FIG. 11 (b) is a front view. As shown in FIG. 11 (a), the adhesive 111 is provided over the fiber-fixing base 109 having a flat upper surface so that the adhesive 111 rises up with surface tension, so that the optical fiber 113 is fixed with the adhesive 111.
In the fixing structure 110 for an optical fiber, only the lower part of the optical fiber 113 is fixed to the fiber-fixing base 109. That is, only one side of the optical fiber 113 is fixed to the fiber-fixing base 109. In this case, if force is given at the rear side of the fixed part of the optical fiber 113 in its axial direction (the direction shown with an arrow G in the drawing), position-shifting occurs in the rotational direction causing the tip of the optical fiber 113 shifting upward or downward (the directions shown with arrows H in the drawing).
Also, as shown in FIG. 11 (b), in such a structure as the optical fiber fixing structure 110, force is given to the optical fiber 113 in the direction toward the fiber-fixing base 109 (the directions shown with arrows I in the drawing) due to the contraction of the adhesive 111 at the time of curing. Thus, it is likely that the position of the optical fiber 113 with regard to the fiber-fixing base 109 may change later after alignment.
Such shifting in the position of an optical fiber becomes a major problem particularly in a case of a single mode lensed fiber. That is, in a semiconductor laser module using an optical coupling system with especially narrow coupling tolerance such as the single mode lensed fiber, such an occurrence of shifting in the position causes a displacement of the optical coupling state between the semiconductor laser and the optical fiber and deteriorates the fiber-end output of the semiconductor laser module.
Particularly in a case in which a wedge lensed fiber with a wedged-shaped tip such as an optical fiber used in an optical coupling with a semiconductor laser having a wavelength range of around 980 nm is used, the optical coupling tolerance in the vertical height direction is extremely narrow due to the optical coupling properties thereof. Therefore, deterioration of the fiber-end output is even a larger problem.
On the other hand, there is a method to fix the optical fiber 113 to the fiber-fixing base by providing a groove or a hole on the fiber-fixing base, disposing the optical fiber 113 in the groove or the hole, and then filling the groove or the hole with adhesive.
FIG. 12 (a) shows a fixing structure for an optical fiber 110a using a fiber-fixing base 109a with a groove having an approximately rectangular cross-section. The fixing structure for an optical fiber 110a has an optical fiber 113 disposed in the groove which is formed on the fiber-fixing base 109a, and adhesive 111 filled into the groove fixes the optical fiber 113 to the fiber-fixing base 109a. 
However, although the optical fiber 113 in the fixing structure for an optical fiber 110a is fixed to the fiber-fixing base 109a in the three directions: left, right, and downward, against the rectangular groove, even so, the optical fiber 113 still receives stress in the right, left, and downward directions (directions shown by arrows J in the drawing) due to the contraction of the adhesive 111 at the time of curing. Therefore, the position of the tip of the optical fiber 113 may be shifted when the adhesive is cured.
Also, FIG. 12 (b) shows a fixing structure for an optical fiber 110b using a fiber-fixing base 109b having a circular hole. The fixing structure for an optical fiber 110b has the optical fiber 113 inserted through the hole formed on the fiber-fixing base 109b and the adhesive 111 filled in the hole fixes the optical fiber 113 to the fiber-fixing base 109b. 
In the fixing structure for an optical fiber 110b, the optical fiber 113 in the hole receives approximately uniform stress in all directions (shown by the arrows K in the drawing) at the time of curing contraction of the adhesive 111. However, the surface of the optical fiber 113 is pulled toward all the directions at the curing contraction of the adhesive 111 and compressive stress is given. Also, temperature change after the adhesion causes expansion or contraction of the adhesive which leads to compressive or tension stress to be given to the whole circumference of the optical fiber 113. In such a case, since whole circumference of the adhesive is restricted by the hole, the stress cannot be released, causing problems such as the adhesive 111 detaching from the fiber-fixing base 109b or the surface of the optical fiber 113, or cracking of the surface of the fiber-fixing base 109a. 